An ion source is a critical component of an ion implanter. The ion source generates an ion beam which passes through the beamline of the ion implanter and is delivered to a semiconductor wafer. The ion source is required to generate a stable, well-defined beam for a variety of different ion species and extraction voltages. In a semiconductor production facility, the ion implanter, including the ion source, is required to operate for extended periods without the need for maintenance or repair.
Ion implanters have conventionally used ion sources with directly heated cathodes, wherein a filament for emitting electrons is mounted in the arc chamber of the ion source and is exposed to the highly corrosive plasma in the arc chamber. Such directly heated cathodes typically constitute a relatively small diameter wire filament and therefore degrade or fail in the corrosive environment of the arc chamber in a relatively short time. As a result, the lifetime of the directly heated cathode ion source is limited.
Indirectly heated cathode ion sources have been developed in order to improve ion source lifetimes in ion implanters. An indirectly heated cathode includes a relatively massive cathode which is heated by electron bombardment from a filament and emits electrons thermionically. The filament is isolated from the plasma in the arc chamber and thus has a long lifetime. Although the cathode is exposed to the corrosive environment of the arc chamber, its relatively massive structure ensures operation over an extended period.
The cathode in the indirectly heated cathode ion source must be electrically isolated from its surroundings, electrically connected to a power supply and thermally isolated from its surroundings to inhibit cooling which would cause it to stop emitting electrons. Known prior art indirectly heated cathode designs utilize a cathode in the form of a disk supported at its outer periphery by a thin wall tube of approximately the same diameter as the disk. The tube has a thin wall in order to reduce its cross sectional area and thereby reduce the conduction of heat away from the hot cathode. The thin tube typically has cutouts along its length to act as insulating breaks and to reduce the conduction of heat away from the cathode.
The tube used to support the cathode does not emit electrons, but has a large surface area, much of it at high temperature. This area loses heat by radiation, which is the primary way that the cathode loses heat. The large diameter of the tube increases the size and complexity of the structure used to clamp and connect to the cathode. One known cathode support includes three parts and requires threads to assemble.
The indirectly heated cathode ion source typically includes a filament power supply, a bias power supply and an arc power supply and requires a control system for regulating these power supplies. Prior art control systems for indirectly heated cathode ion sources regulate the supplies to achieve constant arc current. A difficulty in using a constant arc current system is that, if the beamline is tuned, beam current measured at the downstream end of the beamline can increase either due to the tuning, which increases the percent of current transmitted through the beamline, or due to an increase in the amount of current extracted from the source. Since beam current and transmission are influenced by the same plurality of variables, it difficult to tune for maximum beam current transmission.
A prior art approach that has been utilized in ion sources with directly heated cathodes is to control the source for constant extraction current rather than constant arc current. In all cases where the source is controlled for constant extraction current, the control system drives a Bernas type ion source where the cathode is a directly heated filament.